(a) These two rays are :
(i) Ray of light parallel to principal axis, passes through point F of concave mirror after its reflection.
(ii) Ray of light passing through point C of the mirror, is reflected back on the same path after reflection from the mirror.
Image is erect, virtual and magnified.
(b) `m=-3," "u=-20 cm," "v=?`
`m=(-v)/u" "implies -3=(-v)/(-20)" "implies v=-60 cm`
Distance between the screen and the object distance `=[-60-(-20)]=-40` cm
So, the screen is at a distance of 40 cm from the object.
Minus sign is only as per sign conventions.